[fluider]

It's a war of ratios

1, You chose rear sprocket multilpe times smaller than front sprocket. This configuration with cocentric swingarm pivot is pro-squat. By exchanging them you'd get anti-squat nature.
2, Position of a swingarm pivot makes big difference here. I assume that by moving the swingarm pivot to the output sprocket position the moments created by chain on an idler and rear sprocket can cancel out. Thus making idler's effect negligible and gear ration between front and rear sprockets remains the major factor.

If you can, try to move idler more rearward, farer from swingarm pivot to give it some leverage arm. And also, try to flip front and rear sprockets.

 

[Steve M]

Okay, I thought I would also post and bump this thread as what I have learned is relevant.
The reason I started this thread is because my own bike project featured a swingarm-mounted idler and I had queries regarding the specifics of some published information.
I can now conclude that if an idler is attached to a swingarm and that swingarm is concentric to the front cog, the idler makes no perceptible difference. The following diagram is what I initially thought (based on published literature, I have to say!) was the case, but is NOT;

Suffice to say I am pretty gutted! I made this conclusion based on a test rig that i quickly mocked up. I just wish that I had done this long ago. Well done to those people who figured this out mathematically. I guess I am a pretty graphical kinda guy, and as Socket pointed out I don't think the graphical method transfers to this situation. Oh well!

Here's a pic of the test rig;

Notice the rear cog is fixed to the vertical bar (representing the wheel) and can only move vertically (hence the lower tie-bar). The front cog is connected to a crank-bar and both are free to rotate around the swingarm pivot. The idler is attached but free to rotate ON the swingarm.

Make the idler pulley bigger than the chainring and it'll go the other way

 

[TrueScotsman]

Make the idler pulley bigger than the chainring and it'll go the other way

What, like this.......?

Sorry, Socket- it still gives a pro-squat response.

 

[fluider]

Try this: chainring and rear sprocket of the same size, idler small like derailuer pulley or your orange idler.

 

[TrueScotsman]

Would love to fluider but I have run out of old chainrings!!!! (I found them when tidying out a shed and they happened to be the right size)

I will mock something up, even if it is using the large chainring on the back- this should give a anti-squat response.

 

[Steve M]

aha yes, forgot that'd then make the idler much bigger than the rear cog. Basically what fluider said is correct, the front cog needs to be bigger than the rear one for the idler to have an extending effect, otherwise the extension moment generated by the chain tension between front ring and idler will be smaller than the compressive moment generated by the chain tension between idler and rear cog. I haven't actually run the maths but I suspect the net compressive or extensive moment (without considering tyre tractive force, which in this case will have only a tiny effect since it's almost parallel to your swingarm line) is in fact dependent ONLY on the ratio between front and rear cogs, if the front cog is concentric with the swingarm pivot. Putting an idler in there doesn't change the overall gearing, hence you effectively get two couple moments that add up to the same amount as whatever your unaltered chainline would give with the same gear ratios... exactly as your experiment shows.

 

[TrueScotsman]

Basically what fluider said is correct, the front cog needs to be bigger than the rear one for the idler to have an extending effect,.

Don't you mean "front cog smaller than the rear cog"?

Apart from that. I pretty much agree, when concentric it doesn't matter where an idler is, the net sum of forces cancel each other out so it is dependant on the front and rear cogs size.

 

[Steve M]

Don't you mean "front cog smaller than the rear cog"?

Apart from that. I pretty much agree, when concentric it doesn't matter where an idler is, the net sum of forces cancel each other out so it is dependant on the front and rear cogs size.

Yeah sorry, that's the one!

 

[fluider]

These are snapshots of two consequent pages from 2nd edition of Tony Foale's book.
Page 318

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Page 319

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As you can see, 2nd edition of his book discusses only idlers mounted on the mainframe of entire chasis which is the easier case. However, irrelevant to our topic which is an idler sprocket mounted on the swingarm.

I've done some basic calcs and according to my knowledge the graphical method presented in the 1st edition of his book cannot be right. A very simple example is swingarm cocentrically pivoted with gearbox output sprocket which is of the same diameter as idler sprocket (mounted on swingarm) while idler is located exactly over the sprocket. There the chain between idler and output sprocket creates no torque that could have some effect. Thus only rear sprocket could create some moment. However, graphical method would indicate something very different!

I got an idea of constructing two anti-squat lines based on two segments of chainline divided by idler, then summing them together. But one can easily find other examples where it would indicate something else than calculation would give.

It's funny, now I remember when Gergey Kovacs, the Linkage head programmer, warned me against that graphical method and I was insisting that it must be right and I was begging him to implement it.

Edit: But I remember DW very shortly writing here that if idler is mounted on swingarm then chainline between idler and front sprocket is the one to use for anti-squat projection. Perhaps, he could write something more than 1 sentence and bring a light to my shadowy thoughts...

 

[fluider]

Now I'm watching that there are fewer article extracts available on Foale's web site and I remember the one about anti-squat used to be there as well.